Many of the most popular fresh vegetables -- including crisp green beans, sweet corn and peas -- are in short supply during the early spring. Even when you can find them, prices are likely to be high, and the quality marginal. But the frozen varieties of these and other vegetables can fill in quite nicely -- if prepared creatively.

Creativity is all-important because, when served unembellished, most frozen vegetables are dull at best. But combine them with a tangy cheese sauce, or cook them in a flavorful casserole, and you'll transform them into side dishes that liven up a dinner menu.

There are also times when availability of fresh vegetables is not so much a factor as ease of preparaton. While fresh spinach is marvelous for salads, frozen chopped spinach is simpler to work with when preparing cooked dishes.

Although frozen vegetables do not have the full nutritional value of fresh ones, the freezing process does help preserve their vitamin content. At any rate, they usually are more nutritious than canned vegetables, since the latter are water-packed, causing the vitamins and minerals to dissolve into the water. aBy combining packaged vegetables with eggs, milk or cheese, you can offer your family additional nutrition.

Until the vegetables of summer appear at the peak of freshness, here are some recipes you can try using the frozen products. GREEN BEANS PARISIEN (6 to 8 servings) 2 packages (10 ounces each) frozen French-style green beans 3 tablespoons butter 3 tablespoons flour 2 cups milk 1/4 teaspoon salt 1/2 cup grated Swiss cheese 1/4 cup grated parmesan cheese Dashes of salt, cayenne pepper, nutmeg

Cook green beans in boiling water just until beans separate; drain well, and set aside. In a medium saucepan, over low heat, melt 2 tablespoons of the butter. Blend in flour, and cook, stirring, for 2 minutes; do not let mixture brown. Remove from heat.

Heat milk with the 1/4 teaspoon of salt in another saucepan just to the boiling point. Add hot milk to the butter-flour roux, beating vigorously with a wire whisk. Return the mixture to medium heat and cook, stirring, until the sauce boils. Let boil for 1 minute, and remove from heat. Stir in half of the Swiss cheese and half of the parmesan cheese, and beat well until the cheeses are melted and blended with sauce. Add salt, cayenne pepper and nutmeg to taste.

Spread half the sauce in a lightly buttered 9-by-13-by-2-inch baking dish. Arrange beans on top. Pour rest of sauce over beans, and sprinkle with remaining cheeses. Dot with the remaining tablespoon of butter. (The dish may be assembled in advance up to this point.) Bake, uncovered, in a preheated 375-degree oven for 30 minutes, or until sauce is lightly brown on top.

Note: Almost any cooked frozen vegetable, especially cauliflower or chopped spinach, can be substituted for the green beans. MUSHROOMS FLORENTINE (6 to 8 servings) 2 packages (10 ounces each) frozen chopped spinach, thawed 1/2 cup butter 1 small onion, finely chopped 1 clove garlic, crushed Salt and freshly ground pepper to taste 1 pound fresh mushrooms, sliced 1 cup grated cheddar cheese

Squeeze excess liquid from the thawed spinach, and arrange in the bottom of a lightly buttered gratin dish or shallow casserole. In a small saucepan, melt the butter. Pour half of the butter over the spinach, and sprinkle with the chopped onion, garlic, and salt and pepper to taste. Place half the grated cheese over this. Pour remaining melted butter to a 10-inch skillet, and over medium-high heat, saute the mushrooms until they are soft -- about 5 minutes. Arrange the sauteed mushrooms in the casserole, and sprinkle the remaining grated cheese on top. Bake, uncovered, in a preheated 350-degree oven for 20 minutes, or until the cheese is melted and bubbly. FLUFFY CORN FRITTERS (6 to 8 servings) 1 package (10 ounces) frozen whole kernel corn 1/2 cup milk 1 1/4 cups flour 1 teaspoon salt 1/4 teaspoon freshly ground pepper 2 teaspoons baking powder 1 tablespoon salad oil 2 eggs, well beaten Additional oil for frying

Cook the corn according to package directions; drain well. Place the corn in a medium bowl, and add milk. Sift together the flour, salt, pepper and baking powder and add to the corn mixture. Add oil and eggs, mixing well. In an electric skillet or deep-fat fryer, heat about 1 1/4 cups of oil to 375 degrees. Drop batter by tablespoonfuls into the hot oil, and cook, turning once, until golden-brown on all sides. Remove corn fritters as they cook to drain on paper towels. Serve hot with applesauce, if desired. SWEET & PUNGENT ASPARAGUS SPEARS (4 to 6 servings) 2 packages (10 ounces each) frozen asparagus spears 1/4 cup red wine vinegar 1/2 cup sugar 1/4 cup water 1/2 teaspoon celery seed 1 cinnamon stick 3 whole cloves 1/2 teaspoon salt

One or more days before serving, cook asparagus according to package directions; drain well, and place in a shallow refrigerator dish. Combine the remaining ingredients in a small stainless steel or enamel saucepan, and bring to boiling over medium-high heat; simmer for 5 minutes. Pour hot liquid over asparagus; cover, and refrigerate overnight for longer. Before serving, discard the cinnamon stick and cloves.